They also referred to as control knobs and temperature knobs, they control either the temperature or are used to switch the Saunier Duval boiler on or off and can be found on the front of the boiler. They normally have a shaft that protrudes from the knob directly through the front of the boiler onto the printed circuit board. The shaft when turned, turns a potentiometer that's located on the printed circuit board and if the knob is turned too hard the shaft is designed to break hence protecting the Saunier Duval printed circuit board from damage and an expensive repairs.
Saunier Duval Control knob 801758

Part: 801758

Price: £12.99
Saunier Duval Knob 2000801872

Part: 2000801872

Price: £23.71
Saunier Duval S204698 Control Knob

Part: S204698

Price: £6.31
Saunier Duval Control knob 0020084807

Part: 0020084807

Price: £10.70
Saunier Duval S205146 Control Knob

Part: S205146

Price: £6.57
Saunier Duval S427524 Control Knob

Part: S427524

Price: £12.36
Saunier Duval S204697 Control Knob

Part: S204697

Price: £8.37
Saunier Duval Control knob 2000448323

Part: 2000448323

Price: £9.81